Irregular calcite crystals within a marble. Note the rhombohedral cleavage displayed by the grain just left of center.
Field of View = 2.7 mm, plane light
Extreme inteference colours of calcite and the presence of twinning in the top right grain.
Field of View = 2.7 mm, crossed polars
|Block diagram showing the relationship between the crystallographic axes and the indicatrix axes.|
|Form||variety of habits, but usually coinsist of scalenohedron and rhombohedron combinations. In most rocks calcite forms anhedral grains or grain aggregates|
|moderate negative to high positive, marked
change with stage rotation
nw = 1.658
ne = 1.486
|Cleavage||perfect rhombohedral cleavage, angle between cleavages 74°57'|
extreme, creamy high order colours
|Twinning||lamellar twins parallel to one edge of the cleavage rhomb or along the long diagonal of the rhomb|
|Optic Orientation||extinction is inclined or symmetrical to cleavage traces, the fast ray is parallel to the short diagonal of the rhombohedral faces|
|Composition||dominantly CaCO3, but substitution of Mg, Fe, Mn, or Zn and minor Sr and Ba||Alteration||altered to dolomite during diagenesis, calcite is soluble in natural waters and may be removed by solution|
|Occurrence||common and widespread as a major mineral in limestones, and an accessory in igneous, metamorphic and sedimentary rocks||Distinguishing Features||cleavage, variable relief, extreme interference colours|